Speaker
Description
We present the experimental demonstration of coherent four‑wave mixing in the X‑ray regime (XFWM). We first demonstrate the process in neon near the K‑edge (870 eV) using single broadband SASE pulses from the Swiss Free Electron Laser [1]. A square spatial mask splits the incoming beam into three cut‑out beams arranged in a ‘folded BOX’ geometry, enabling background‑free signal detection. The beams intersect in a neon gas cell at a pressure of several hundred millibars, generating multi‑color resonant nonlinear signals involving Raman transitions, including X‑ray coherent anti‑Stokes (X-CARS) processes, which are characterized with an inline photon spectrometer. A rate‑equation model supports the interpretation of the measured XFWM signal. The extension of this experiment to molecular samples (O2, CO2 and N2O) addressing the K edges of O and N at 400 and 530 eV is also demonstrated, confirming the applicability of the method to molecular systems. In addition, we introduce approaches to extend the methodology to the time domain using the two‑colour mode available at the Athos branch of SwissFEL. In these preliminary experiments, we demonstrate the capability to generate time‑resolved signals arising from controlled stimulated emission in O2 following a Raman transition.
Finally, we discuss the perspectives to extend the discussed methodology to the attosecond regime, in line with a recent attosecond X-ray pump-attosecond X-ray probe experiment performed at the SQS instrument of EuXFEL. In this experiment, attosecond X‑ray interferometry was used to generate and probe site‑specific electronic wavepackets in acetic acid. By selectively exciting the two dominant pre‑edge resonances below the oxygen K‑edge with attosecond pump–probe pulses in a collinear transient‑absorption geometry, site‑selective bond excitation is demonstrated with attosecond resolution. The robustness of the setups, the strength of the signals, and the combined spectral, temporal and spatial information achieved in these experiments represent a significant step forward to establish a pathway towards multidimensional spectroscopy in the X‑ray domain.
References: [1] Morillo-Candas, et al. (2026) Nature, 649, 590–596
| Scientific Topics | AMO Physics |
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